1. Field of the Invention
The present invention relates to communication systems, and, more particularly, to digital subscriber lines using error correction.
2. Description of the Related Art
Digital subscriber line (DSL) technology dramatically increases the digital capacity of ordinary, local-loop telephone lines. Although DSL technologies are still emerging, there are a multitude of versions of DSL transmission technology. DSL allows for normal voice use of the xe2x80x9cplain old telephone servicesxe2x80x9d (POTS) coupled with concurrent high-speed data transmission over the same lines. DSL data can share the same line as typical voice telephone communications, because the data uses higher frequencies than the voice band.
One DSL technology is Asymmetrical DSL (ADSL). ADSL is available in two modulation schemes: discrete multitone (DMT) or carrierless amplitude phase (CAP). ADSL DMT systems are often referred to as multicarrier systems because DMT uses many narrow-band carriers, all transmitting simultaneously in parallel.
ADSL systems using DMT split the available bandwidth into a number of discrete subchannels, also called bands or bins. Each subchannel carries a portion of the total information being transmitted. The many subchannels are independently modulated with a carrier frequency corresponding to the center frequency of the subchannel and are processed in parallel. DMT can allocate data so that the throughput of every single subchannel is maximized. For example, DMT can maximize channel throughput by sending different numbers of bits on different subchannels. The number of bits on each subchannel depends on the Signal-to-Noise Ratio (SNR) of each subchannel. If a subchannel can not carry any data, it can be turned off, and the use of available bandwidth is optimized.
ADSL can be seen as a frequency-division multiplexed (FDM) system in which the available bandwidth of a single copper loop is divided into three parts: POTS voice band, upstream data channels and downstream data channels. In ADSL DMT-systems, the downstream channels are typically divided into 256 4-kHz-wide subchannels, and the upstream channels are typically divided into 32 subchannels. A baseband can be occupied by the POTS voice service.
In addition to choosing a robust modulation technique, the performance of a DMT system can be further increased by using error-control coding techniques. Error-control coding techniques can increase data capacity and protect against noise and interference. By adding redundancy it is possible to increase the efficiency of a communication channel. Different coding techniques have different benefits such as improved performance for the same signal power (or keeping the same performance at lower power), improving the ability to cope with noise, maximizing throughput, or ensuring a consistent quality of service.
Error correction characterization typically takes place in systems employing some or all of: fixed rate transfers, fixed error correction schemes, single carrier transmission, and adjustable sample rates. The characteristics of such systems allow one to predict the costs and gains of coding schemes. Traditional coding gain techniques exist for single carrier systems that increase transmission bandwidth to account for redundancy (overhead) bits (e.g., Reed-Solomon coding). Bandwidth efficient schemes such as Trellis coding that increase constellation size instead of speeding up the transmission rate also exist. Such techniques typically assume a fixed energy/bit (which yields a predictable coding cost), variable transmission bandwidth and fixed error-control coding parameters. For these types of systems, it is possible to predict the net coding gain, which is the net effect of the overhead bits (cost) and performance improvement (gain). Since most systems have fixed coding schemes the net gain is known a priori.
But for a multirate system which adapts to variable line conditions with a fixed sample rate, the coding gains and costs are different for each line. In fact, with a multirate, multicarrier system such as ADSL, there has been no known method for predicting the gains and costs of employing error correction. Specifically, it has not been known how to accurately predict the cost of overhead bits in a multicarrier system. Capacity or system performance estimation (measured with margin) based on net gain has therefore been inaccurate. Furthermore, the error correction schemes implemented in a system such as ADSL are programmable and can be configured to yield different coding gains. A poor error correction configuration can hurt system performance, and reduce the amount of user payload capacity.
There is a need for a method and apparatus that allows one to determine which configuration (e.g., coding or error correction configuration) provides an optimal coding gain performance for any given line or set of line characteristics including variable energy/bit, fixed bandwidth, and variable error-control coding parameters. Specifically, for implementation of an ADSL system, there is a need for a method to select the best system configuration based on multicarrier SNR values and the different coding gains related to varied parameters of the error correction schemes.